Respondent Driven Sampling (RDS) is arguably the best and most common method being used to survey "hidden" populations that are hard to sample using standard probability methods. Examples include injection drug users (IDU), men who have sex with men (MSM), and female sex workers. Infections amongst just two hidden populations (MSM, IDUs) accounted for the estimated 69% of 53,600 new HIV infections in the US during 2006. We propose to develop statistical methods for improving estimation in RDS samples with a focus on developing tools and methods that will help, and be accessible by, researchers in the field. From a statistical perspective, RDS is an adaptive sampling regime aimed at obtaining a probability sample. It is often effective at acquiring a sample, but the degree to which it can be considered a probability sample, with known inclusion probabilities, is unclear. The current estimators of these inclusion probabilities are known to be problematic. We will develop open-source user-friendly statistical software for RDS incorporating state-of-the-art methods, hold workshops to establish collaboration between applied and methodological researchers, disseminate these new methods and provide training using the tools developed. This project is significant because it will result in major advances in methodology for hidden population measurement and related scientific problems. The investigators are uniquely qualified as the first statisticians with close collaborative ties with field-researchers to systematize statistical understanding of RDS. The project is innovative in that it challenges the existing paradigm of RDS inference and proposes a new approach based on cutting edge statistical ideas and models. There is a dearth of statistical methodology justifying RDS. This project will produce a systematic statistical framework within which to understand the strengths and weaknesses of RDS. In the proposed work we will represent the complexities of the RDS procedure, but also allow the uncertainty of the resulting inference to be quantified. The development of statistical methodology for RDS is of vital importance to the social and behavioral sciences. We will disseminate the methodology for estimation, diagnostics and quantification of uncertainty via open-source user-friendly software aimed at field researchers. These will be applicable to both future RDS data, and the large existing data bases of RDS surveys. PUBLIC HEALTH RELEVANCE: Respondent Driven Sampling (RDS) is arguably the best and most common method being used to survey "hidden" populations that are hard to sample using standard probability methods. Examples include injection drug users (IDU), men who have sex with men (MSM), and female sex workers. Infections amongst just two hidden populations (MSM, IDUs) accounted for the estimated 69% of 53,600 new HIV infections in the US during 2006 (CDC report, Hall, 2008). The purpose of this grant is to improve these kinds of estimates as current estimation practices are known to be problematic.